<!DOCTYPE html>

<div class="container">
    <div class="left">left</div>
    <div class="right">
        <div width="100%">asjhdkajs</div>
    </div>
</div>

<style>
    * {
        margin: 0;
        padding: 0;
    }
    
    .container {
        display: flex;
        width: 100%;
        height: 100%;
        border: 1px solid blue;
    }
    
    .container .left {
        flex: 0 0 200px;
        /* 不伸缩，固定宽度为200px */
        height: 100%;
        background-color: aqua;
        border: 1px solid black;
    }
    
    .container .right {
        flex: 1;
        /* 伸缩，占据所有剩余空间 */
        height: 100%;
        background-color: bisque;
        border: 1px solid red;
    }
</style>